AE10.pdf

Hanger noise like the 45 kg 10 lb ball impact shows

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Hanger noise, like the 4.5 kg (10 lb) ball impact, shows up as high amplitude, large magnitude acoustic emission signals that can travel the entire length of the pipeline. Because there is no effective way to keep from recording this emission, the best that can be done is to calculate the approximate location of the high amplitude emission and to compare it to hanger locations and external penetrations in the piping. Standard acoustic emission data analysis used on pressure vessels depends on independent channel information to determine the presence of a discontinuity. This technique, which considers the data acquired by each channel as if that channel existed independently in space, works very well when background noise is not a problem and the loading can be highly controlled . The results obtained from previous acoustic emission tests performed on hot reheat systems have not been good when independent channel analysis has been used. 345 Electric Power Applications of Acoustic Emission Testing
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The independent channel analysis technique depends on a discontinuity showing increasing emission magnitude in response to the external loading imposed on the test object. The problem encountered when using this technique on a steam line is that the steam flow noise can increase, decrease or remain steady while the pipe is changing temperature. If the steam flow emission is included in the analysis, the results can be unreliable. Analysis procedures should include both filtering of spatial events and correlation of event characteristics . Spatial filtering, also known as source location, looks at emission that is large enough in magnitude to strike at least two adjacent transducers on the pipe, in a time frame dictated by the distance between the transducers and by the speed of sound in the material. Sound travels about 3660 m·s –1 (12 000 ft·s –1 ) in this pipe. Spatial filtering eliminates a large part of the random emission generated by the steam flow because this steam flow emission usually does not strike two adjacent transducers in the required time frame. Even with spatial filtering, if enough random emission strikes the transducers, eventually the system will find some events that show initial locations. Event characteristic correlation is a technique where each locatable event is plotted on a scatter plot showing the duration of the event compared to its amplitude. This technique discriminates between emission from steam flow, mechanical noise and discontinuity growth indications. Generally speaking, emissions from typical sources detected during a steam line test show the following characteristics. 1. Steam flow emission is short in duration and very low in amplitude, just above the detection threshold.
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  • Fall '19
  • Test, Nondestructive testing, Acoustic Emission, Acoustic Emission Testing

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